Flow measuring device

ABSTRACT

A device is used for measuring the flow of media flowing in conduits, particularly of breathing gas directed to and/or from a patient in respirators. The device is particularly characterized by actuating means for setting the device in a zero position when the passage of the flow is zero.

United States Patent [191 Olsson [4 1 Oct. 16, 1973 5.4. OW .MEASPBIEEBEY!9PW- W I. n [75] Inventor: Sven-Gunnar Sigvard Olsson,

Sollentuna, Sweden [73] ignee: Siemens Alrtiengesellscliaft, I I

Erlangen, Germany 22] Filed: "fifiil'fiflfii [21] Appl. No.: 126,549

[30] Foreign Application Priority Data Mar. 25, 1970 Sweden 4096/70 [52]US. Cl 73/194 R, 73/228, 128/208 [51] Int. Cl. Golf U011 [58] Field ofSearch 73/1, 194 E, 194 R,

73/228; 128/208, 142, 142.4, 188, DIG. 17

[56] References Cited UNITED STATES PATENTS 3,424,000 l/1969 Chelner etal. 73/228 FOREIGN PATENTS OR APPLICATIONS 1,508,303 11/1967 France128/208 OTHER PUBLICATIONS Melville, A. W. et al., A Servo OperatedRespirator for Premature Infants, IRE Transactions on MedicalElectronics, Dec. 1958, p. 75-81.

Primary Examiner-Richard C. Queisser Assistant Examiner--John P.Beauchamp Att0rneyRichards & Geier 5 7] ABSTRACT A device is used formeasuring the flow of media flowing in conduits, particularly ofbreathing gas directed to and/or from a patient in respirators. Thedevice is particularly characterized by actuating means for setting thedevice in a zero position when the passage of the flow is zero.

3 Claims, 1 Drawing Figure Pmimtnncnsm 3765239 I N VENTOR SVEN-GUN NARSlGVAR D OLSSON FLOW MEASURING DEVICE This invention relates to a devicefor measuring the flow of media flowing in conduits, particularly ofbreathing gas directed to and/or from a patient in respirators.

When a respirator is used it is desirable to measure the gas flow toand/or from a patient so as to be able to control whether the amount ofgas breathed out by the patient is equal to the amount of gas providedby the respirator. From such measurements can be determined, forexample, whether there is a leak in the conduits between the respiratorand the patient. in the case of respirators with servo-actuation themeasurement can be also used as a regulating value for keeping constanta desired flow. To provide convenient indication or further actuation ofthe result of measuring, the measurement should consist of an electricalsignal.

A fiow measurer suitable for the measuring of breathing gas flow must beconstructed by taking into consideration that the pressure drop in themeasurer is very small and in the case of a flow of l L/sec. sec. shouldif possible not exceed the value 1 cm H O. Flow measurers are knownwhich fulfill these requirements and at the same time are comparativelycheap. However, these cheap flow measurers have the drawback that theyare comparatively unstable with respect to zero shifting due to thenecessarily required very high amplification. Other known flow measurerswhich are less unstable in this respect are considerably more expensiveand more complicated. However, for flow measurers of respirators it isabsolutely necessary that the zero shifting should be small.Furthermore, these flow measurers must have a wide measuring range. Inaddition, measuring errors which sometimes occur should be held as smallas possible in small flow values as well.

' An object of the present invention is the provision of a device of thedescribed type wherein there is no shifting of the zero position, sothat there can be practically no more measuring errors; this has alsothe advantage that simple and cheap flow measurers can be used withoutdetrimentally affecting the entire function of the device.

Other objects of the present invention will become apparent in thecourse of the following specification.

In the accomplishment of the objectives of the present invention it wasfound desirable to provide actuating means for setting the zero positionof the device whenever there is a zero passage of the flow. This zeroflow passage is preferably determined by an indicator which is separatefrom the actual flow measurer.

The present invention utilizes the fact that the breathing-in flow aswell as the breathing-out flow are equal to zero during a part of thebreathing cycle. If the device is set to zero each time when thebreathing cycle passes through zero, then the zero error can never begreater than that which takes place during a single breathing cycle evenwhen there are several cycles which follow each other. Thus the deviceof the present invention has an advantage over prior art devices whereinin the case of several breathing cycles which follow each other theappearing total zero error is the sum of zero errors which took place inthe individual breathing cycles.

According to a preferred embodiment of the present invention the settingof the zero position takes place with the closing of a valve regulatingthe flow. This makes certain that the device will have a zero positionat the time when the flow is actually zero.

In a device according to the present invention which produces anelectrical signal corresponding to the size of the flow, the zerosetting takes place preferably in that the signal, if necessary afterprevious amplification, is transmitted through a condenser to anamplifier of high-inlet impedance and the amplification inlet is placedat the zero potential whenever there is a closing of the valve dependingupon the switching means operated by valve movement. The operation ofthe switching means in order to supply the zero potential to theamplifier inlet takes place advantageously by means of an actuatingimpulse which is produced by a photocell device sensing the valvemovement during the closing of the valve.

Advantageously a field effect transistor is used as the switching means.

The invention will appear more clearly from the following detaileddescription when taken in connection with the accompanying drawing thesole FIGURE of which is a diagram showing the main circuit connectionsof the device of the present invention.

The Drawing shows an actuating valve 1 and a flow measurer 2. The valve1 regulates the flow of the flowing medium in a conduit 3, while theflow measurer 2 is used to measure the strength of the flow passingthrough the conduit 3.

The conduit 3 consists preferably of an in-breathing and out-breathingconduit of a respirator having servoactuation. Thus the valve 1 and themeasurer 2 are used for actuating and measuring the flow of breathinggas in the conduit 3.

The flow measurer 2 consists substantially of a resistance strain gaugeintroduced into the conduit 3, the strain gauge being bent dependingupon the strength'of the flowing breathing gas. Each bending of theresistance strain gauge produces a change in resistance of a resistancebridge circuit. An electrical signal produced at the outlet of thebridge circuit due to change in resistance is then always proportionalto the strength of flow.

The resistance strain gauge must be of small size to keep low pressuredrop over the strain gauge in the conduit 3. Thus a comparatively weaksignal is produced which must be amplified in order to be subjected tofurther operations. This amplification takes place in an amplifier 4connected to the outlet of the flow measurer 2.

The amplified signal is then transmitted through a condenser 5 to afurther amplifier 6 having a higher inlet impedance. The signalappearing at the outlet of this amplifier 6 can be either transmitteddirectly to an indicating device for indicating the strength of flow orit can be used for servo-actuation of the respirator.

The inlet of the amplifier 6 can be connected with zero potential(ground) by a line 7 as well as by a field effect transistor 8 servingas a switch. The actuation of the transistor 8 takes place dependingupon the movement of the valve 1, by a photo-cell device 9 which sensesthe valve movement.

The operation of the device of the present invention is as follows:

Tensions take place in the pressure strain gauge due to temperaturevariations or, for example, changes in the position of the strain gaugein the conduit 3, which cause a zero shifting at the outlet of the Howmeasurer 2. Temperature variations or, for example, position changes ofthe strain gauge thus may cause an indication of a flow value at the endof each breathing cycle although actually there is no flow.

The influence of such zero position errors upon the measured result isavoided by placing the transistor 8 into conducting condition each timeduring zero position of the flow, namely, connecting the inlet of theamplifier 6 to the zero potential. The time for the switching on of thetransistor 8 is fixed by an actuating impulse which is produced by thephoto-cell device 9 at the moment when the actuating valve closes,namely, when the flow is zero.

During the time period when the inlet of the amplifier 6 is connected tothe zero potential, the condenser receives the voltage value whichcorresponds to the zero error. When the transistor 8 is switched outagain no substantial change in the charge of the condenser 5 takes placedue to the high inlet impedance of the amplifier 6. Thus the voltagecorresponding to a zero point shifting at the outlet of the firstamplifier 4 does not affect any more the inlet of the amplifier 6, sothat at the outlet of the amplifier 6 there is only a signal which isproportional to the momentary value of the flow in the conduit 3.

If there is a further shifting of the zero position in one of thefollowing breathing cycles, this shifting will have no effect sinceafter the following closing of the transistor 8 the condenser 5 willreceive this new value and thus this new zero point shifting cannot haveany effect upon the amplifier 6.

The present invention results in that an eventually appearing zero pointerror cannot proceed to indication or to further treatment any more, sothat erroneous measuring results or erroneous operation of therespirator are avoided. Since every measuring result is independant fromzero point errors, comparatively simple and cheap flow measurers can beused. This greatly simplifies the entire structure of the device of thepresent invention and its costs are considerably lowered.

I claim:

1. A device for measuring the flow ofa medium, such as breathing gas,said device comprising a conduit for the medium, an actuating valvelocated in the conduit and regulating the flow of the flowing medium inthe conduit, and a flow measurer located in the conduit and producing anelectrical signal, said signal being a measure for the strength of theflow passing through the conduit, said device further comprising acapacitor connected to the signal-output of said flow measurer, asignal-amplifier connected to said capacitor and having a high-ohmicinput impedance, switching means connected to the input of saidamplifier, and actuating means connected to said switching means and tosaid valve, said actuating means operating said switching meansdepending on the movement of said valve to place the amplifier input atzero potential when the valve is closed.

2. A device in accordance with claim 1, wherein the actuating meanscomprise a photo-cell device sensing the valve movement and producing anactuating impulse for the switching means when the valve closes.

3. A device in accordance with claim 2, wherein said switching meanscomprise a field effect transistor actuated by the actuating impulse ofthe photo-cell device. k

1. A device for measuring the flow of a medium, such as breathing gas,said device comprising a conduit for the medium, an actuating valvelocated in the conduit and regulating the flow of the flowing medium inthe conduit, and a flow measurer located in the conduit and producing anelectrical signal, said signal being a measure for the strength of theflow passing through the conduit, said device further comprising acapacitor connected to the signal-output of said flow measurer, asignal-amplifier connected to said capacitor and having a high-ohmicinput impedance, switching means connected to the input of saidamplifier, and actuating means connected to said switching means and tosaid valve, said actuating means operating said switching meansdepending on the movement of said valve to place the amplifier input atzero potential when the valve is closed.
 2. A device in accordance withclaim 1, wherein the actuating means comprise a photo-cell devicesensing the valve movement and producing an actuating impulse for theswitching means when the valve closes.
 3. A device in accordance withclaim 2, wherein said switching means comprise a field effect transistoractuated by the actuating impulse of the photo-cell device.